AbstractIt is quite appealing to base the description of pattern-based searches on positive as well as negative conditions. We would like for example to specify that we search for white cars that are not station wagons. To this end, we define the notion of anti-patterns and their semantics along with some of their properties. We then extend the classical notion of matching between patterns and ground terms to matching between anti-patterns and ground terms. We provide a rule-based algorithm that finds the solutions to such problems and prove its correctness and completeness. Anti-pattern matching is by nature different from disunification and quite interestingly the anti-pattern matching problem is unitary. Therefore the concept is appropriate to ground a powerful extension to pattern-based programming languages and we show how this is used to extend the expressiveness and usability of the Tom language.